Dose-controlled, floating evaporative assembly of aligned carbon nanotubes for use in high performance field effect transistors

ABSTRACT

High density films of semiconducting single-walled carbon nanotubes having a high degree of nanotube alignment are provided. Also provided are methods of making the films and field effect transistors (FETs) that incorporate the films as conducting channel materials. The single-walled carbon nanotubes are deposited from a thin layer of organic solvent containing solubilized single-walled carbon nanotubes that is spread over the surface of an aqueous medium, inducing evaporative self-assembly upon contacting a solid substrate.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under 1129802 and0520527 awarded by the National Science Foundation. The government hascertain rights in the invention.

BACKGROUND

Single-walled carbon nanotubes (SWCNTs) are key building blocks fornanoscale science and technology due to their interesting physical andchemical properties. SWCNTs are particularly promising for high speedand low power semiconductor electronics. A challenge, however, is thehierarchical organization of these building blocks into organizedassemblies and, ultimately, useful devices. Ordered structures arenecessary, as random network SWCNT thin films result in sub-optimalelectronic properties including reduced channel conductance andmobility. Numerous techniques for aligning SWCNTs have been explored tosolve this shortcoming and achieve higher conductance and mobility.These approaches can be divided into two main categories: (a) directgrowth via chemical vapor deposition and arc-discharge, and (b) postsynthetic assembly. In the case of direct growth, both metallic andsemiconducting SWCNTs are produced. In this case, the performance ofSWCNT field effect transistors (FETs) is limited by the metallic SWCNTs(m-SWCNTs), thus motivating attempts to purify semiconducting SWCNT(s-SWCNT) samples with homogeneous electronic properties.

A variety of post-synthetic sorting methods have been developed toseparate m- and s-SWCNTs according to their specific physical andelectronic structures, which are usually implemented in aqueous ororganic solutions. In order to take advantage of the high purity ofs-SWCNTs that can be produced by these solution-based sorting approachesin semiconductor electronic devices, solution-based methods forassembling and aligning s-SWCNTs, such as evaporation-drivenself-assembly, blown-bubble assembly, gas flow self-assembly,spin-coating, Langmuir-Blodgett and -Shafer methods, contact-printingassembly, and AC electrophoresis, have been developed. (See, Shastry, T.A.; Seo, J. W.; Lopez, J. J.; Arnold, H. N.; Keller, J. Z.; Sangwan, V.K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Large-area,electronically monodisperse, aligned single-walled carbon nanotube thinfilms fabricated by evaporation-driven self-assembly. Small 2013, 9,45-51; Druzhinina, T.; Hoeppener, S.; Schubert, U. S. Strategies forPost-Synthesis Alignment and Immobilization of Carbon Nanotubes. Adv.Mater. 2011, 23, 953-970; Yu, G.; Cao, A.; Lieber, C. M. Large-areablown bubble films of aligned nanowires and carbon nanotubes. Nat.Nanotechnol. 2007, 2, 372-7; Wu, J.; Jiao, L.; Antaris, A.; Choi, C. L.;Xie, L.; Wu, Y.; Diao, S.; Chen, C.; Chen, Y.; Dai, H. Self-Assembly ofSemiconducting Single-Walled Carbon Nanotubes into Dense, Aligned Rafts.Small 2013, 9, 4142; LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y.W.; Kim, J. M.; Bao, Z. Self-sorted, aligned nanotube networks forthin-film transistors. Science 2008, 321, 101-4; Cao, Q.; Han, S. J.;Tulevski, G. S.; Zhu, Y.; Lu, D. D.; Haensch, W. Arrays of single-walledcarbon nanotubes with full surface coverage for high-performanceelectronics. Nat. Nanotechnol. 2013, 8, 180-6; Jia, L.; Zhang, Y.; Li,J.; You, C.; Xie, E. Aligned single-walled carbon nanotubes byLangmuir-Blodgett technique. J. Appl. Phys. 2008, 104, 074318; Liu, H.;Takagi, D.; Chiashi, S.; Homma, Y. Transfer and alignment of randomsingle-walled carbon nanotube films by contact printing. ACS Nano 2010,4, 933-8 and Shekhar, S.; Stokes, P.; Khondaker, S. I. Ultrahigh densityalignment of carbon nanotube arrays by dielectrophoresis. ACS Nano 2011,5, 1739-46.) While each of these methods has its strengths, new methodsare still needed to improve the fidelity of s-SWCNT assembly andalignment in order to enable the fabrication of practical s-SWCNT-basedelectronic devices.

SUMMARY

High density films of s-SWCNTs having a high degree of nanotubealignment are provided. Also provided are methods of making the filmsand field effect transistors that incorporate the films as conductingchannel materials.

One embodiment of a method of forming a film of aligned s-SWCNTs on asubstrate includes the steps of: (a) partially submerging a hydrophobicsubstrate in an aqueous medium; (b) applying a dose of a liquid solutionto the aqueous medium, the liquid solution comprisingsemiconductor-selective-polymer-wrapped s-SWCNTs dispersed in an organicsolvent, whereby the liquid solution spreads into a layer on the aqueousmedium at an air-liquid interface andsemiconductor-selective-polymer-wrapped s-SWCNTs from the layer aredeposited as a stripe of aligned semiconductor-selective-polymer-wrappeds-SWCNTs on the hydrophobic substrate; and (c) at least partiallywithdrawing the hydrophobic substrate from the aqueous medium, such thatthe portion of the hydrophobic substrate upon which the stripe ofaligned semiconductor-selective-polymer-wrapped s-SWCNTs is deposited iswithdrawn from the air-liquid interface. Steps (b) and (c) may berepeated one or more times in sequence to deposit one or more additionalstripes of aligned semiconductor-selective-polymer-wrapped s-SWCNTs onthe hydrophobic substrate.

An embodiment of a film comprising aligned s-SWCNTs, can becharacterized in that the s-SWCNTs in the film have a degree ofalignment of about ±15° or better and the single-walled carbon nanotubelinear packing density in the film is at least 40 single-walled carbonnanotubes/μm. In some embodiments, the films have a semiconductingsingle-walled carbon nanotube purity level of at least 99.9%.

An embodiment of a field effect transistor includes: a source electrode;a drain electrode; a gate electrode; a conducting channel in electricalcontact with the source electrode and the drain electrode, theconducting channel comprising a film comprising aligned s-SWCNTs,wherein the s-SWCNTs in the film have a degree of alignment of about±15° or better and the single-walled carbon nanotube linear packingdensity in the film is at least 40 single-walled carbon nanotubes/μm;and a gate dielectric disposed between the gate electrode and theconducting channel. Some embodiments of the transistors have anon-conductance per width of at least 5 μS μm⁻¹ and an on/off ratio perwidth of at least 1×10⁵.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A schematic illustration of the iterative process used tofabricate a film of s-SWCNTs, driven by the spreading and evaporation ofcontrolled doses of organic solvent at an air/water interface.

FIG. 2. Optical microscope image of narrow films (stripes) of s-SWCNTs.

FIG. 3. High resolution SEM image of stripes of s-SWCNTs.

FIGS. 4(A) and (B) AFM images of stripes of s-SWCNTs.

FIG. 5. Raman spectra of arc-discharge SWCNTs with an incident 532 nmlaser.

FIG. 6 shows the G-band of s-SWCNT arrays taken at various angles (0°,10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, 90°, from back to front) as afunction of the angle between the laser polarization and the s-SWCNTstripe long-axis. Inset: angular dependence of the Raman intensity at1,594 cm⁻¹.

FIG. 7 shows the probability distribution function showing the degree ofalignment of arc-discharge and HiPco s-SWCNTs.

FIG. 8 shows the comparison of dose-controlled, floating evaporativeself-assembly aligned SWCNTs (this work) with other methods reported inthe literature. CVD growth was used for the circles (metallic andsemiconducting SWCNT are mixed) and post synthesis assembly was utilizedfor the squares.

FIG. 9. Output characteristics at varying gate voltages demonstratingthe p-type behavior of a s-SWCNT film in a typical FET (L=9 μm, w=4 μm).

FIG. 10. Transfer characteristics demonstrating a on/off ratio ofgreater than 10⁶ and high current output for the typical FET.

FIG. 11. SEM image of an aligned s-SWCNT-based transistor. Scale bar is2 μm.

FIG. 12. SEM image (scale bar=200 nm) of a 400 nm channel length FET.

FIG. 13. AFM image of an aligned s-SWCNT stripe. The inset is the filmthickness profile along the dark grey cross-section.

FIG. 14. Raman map of a ˜20 μm² stripe overlaying a corresponding SEMimage (scale bar=1 μm). Grayscale bar indicates G-band intensity dividedby Si intensity.

FIG. 15. Output characteristics of a 9 μm channel length FET atV_(GS)=−30 V (squares) to 10 V (diamonds) in 10 V increments. The insetshows the forward and backward sweeps of transfer curves at K_(DS)=−1 V.

FIG. 16. Transfer characteristics of 22 different 400 nm channel lengthFET devices at V_(DS)=−1 V. The inset is the on/off ratio histogram.

FIG. 17. On- (squares) and off- (circles) conductance at each channellength. Median values are plotted, and the error bars indicate maximumand minimum values.

FIG. 18. Comparison of s-SWCNT FET performance (on-conductance per widthversus conductance modulation) achieved in Example 2 to previousstudies. Median values for this work are plotted as stars and calculatedfrom the following number of devices at each channel length: 400 nm (22devices), 1-2 μm (6 devices), 3-4 μm (6 devices), and 9 μm (4 devices).Error bars for this work indicate maximum and minimum values.

DETAILED DESCRIPTION

High density films of s-SWCNTs having a high degree of nanotubealignment are provided. Also provided are methods of making the filmsand field effect transistors that incorporate the films as conductingchannel materials.

The films are formed using a method referred to in this disclosure as“dose-controlled, floating evaporative self-assembly”. This method usesa thin layer of organic solvent containing solubilized s-SWCNTs at anair-liquid interface to deposit films of aligned s-SWCNTs on a partiallysubmerged hydrophobic substrate. The method decouples the s-SWCNT filmformation from the evaporation of a bulk liquid medium and, byiteratively applying the s-SWCNTs in controlled “doses”, allows for therapid sequential deposition of a series of narrow s-SWCNTs films, or“stripes”, with continuous control over the width, s-SWCNT density andperiodicity of the stripes. The resulting films can be characterized bya high degree of s-SWCNT alignment and high s-SWCNT densities. As aresult, they are well suited for use as channel materials in FETs havinghigh on-conductance values and high on/off ratios.

An advantage of the dose-controlled, floating evaporative self-assemblymethod is that it allows for the deposition of s-SWCNTs with exceptionalelectronic-type purity—sorted using semiconductor-selective polymers—inorganic solvents. Unlike anionic surfactants, which have been used tosort s-SWCNTs in aqueous solution, semiconductor-selective polymers areadvantageous because they can sensitively and selectively “pick out”semiconducting nanotubes directly during dispersion from raw SWCNTpowders, thereby avoiding the need for subsequent post-dispersionsorting.

An embodiment of the dose-controlled, floating evaporative self-assemblymethod is illustrated schematically in FIG. 1. As shown in panel (i) ofthe figure, the method begins with a hydrophobic substrate 102 partiallysubmerged in an aqueous liquid medium 104, such as water. A dose of aliquid solution in the form of a droplet 106 is dropped into liquidmedium 104, preferably in close proximity to substrate 102. The liquidsolution, which is also referred to herein as an “organic ink” or a“s-SWCNT ink”, comprises s-SWCNTs 108 dispersed in an organic solvent110. The s-SWCNTs have a semiconductor-selective polymer coated on theirsurfaces and are referred to herein as“semiconductor-selective-polymer-wrapped” s-SWCNTs. The liquid solutionspreads (represented by solid arrows in the figure) into a thin layerover the surface of aqueous liquid medium 104 at the air-liquidinterface (panel (ii)). Driven by diffusion,semiconductor-selective-polymer-wrapped s-SWCNTs 108 in the thin layerof liquid solution come into contact with and deposit onto hydrophobicsubstrate 102 as a stripe 112 of aligned s-SWCNTs near the air-liquidinterface, while organic solvent 110 rapidly evaporates. Stripe 112spans the width of the substrate. Without intending to be bound to aparticular theory, the inventors believe that the s-SWCNTs adopt analigned configuration because, as the solvent level rapidly decreasesduring evaporation, the s-SWCNTs tend to orient perpendicular to theevaporation front because that is sterically a more favored position. Itis also likely that there is less of a steric penalty for the s-SWCNTsto turn sideways, rather than for them to stand up out of the solventlayer into the air while the solvent front recedes.

Once stripe 112 has been formed, substrate 108 can be elevated such thatthe stripe is withdrawn from the air-liquid interface (panel (iii)).Additional doses of the liquid solution can be added sequentially andthe process repeated to form a series of stripes 114 comprising aligneds-SWCNTs (panel (iv)). Using this process, very thin films ofs-SWCNTs—typically having a thickness of only a monolayer or a bilayerof s-SWNCTs—can be deposited.

Optionally, the semiconductor-selective polymer can be partially orentirely removed from the s-SWCNTs after stripe formation. This can beaccomplished, for example, using a polymer-selective dry or wet chemicaletchant or through selective thermal decomposition of the polymer. Insome embodiments of the methods, the amount of semiconductor-selectivepolymer on the s-SWCNTs can be reduced prior to adding them to the dose.

By controlling the velocity of the withdrawal of substrate 108, thestripe width (i.e., the dimension of the stripe that runs parallel tothe direction of withdrawal), stripe periodicity and s-SWCNT density ofthe stripes can be carefully controlled. The optimal substratewithdrawal rate can depend on a variety of factors, including thedesired characteristics of the final deposited films, the nature of thesubstrate and/or the rate of dose dispensation. The present methods areable to deposit stripes over a large substrate surface area rapidly,even at room temperature (about 23° C.) and atmospheric pressure. Forexample, in some embodiments, the methods deposit stripes of aligneds-SWCNTs at a substrate withdrawal rate of at least 1 mm/min. Thisincludes embodiments in which the substrate withdrawal rate is at least5 mm/min. By way of illustration, using such high withdrawal rates, thepresent methods are able to deposit a series of stripes of aligneds-SWCNTs with a periodicity of 200 μm or less over the entire surface ofa 12 inch wafer (e.g., Si wafer) in less than one hour.

The density of SWCNTs in the stripes refers to their linear packingdensity, which can be quantified in terms of the number of SWCNTs per μmand measured as demonstrated in Example 1, below. In some embodiments,the dose-controlled, floating evaporative self-assembly method depositsfilms having a SWCNT density of at least 30 SWCNTs/μm. This includesembodiments in which the films have a SWCNT density of at least 35SWCNTs/μm, at least 40 SWCNTs/μm, at least 45 SWCNTs/μm and at leastabout 50 SWCNTs/μm.

The degree of alignment of the SWCNTs in the stripes refers to theirdegree of alignment along their longitudinal axes within a stripe, whichcan be quantified as demonstrated in Example 1, below. In someembodiments, the dose-controlled, floating evaporative self-assemblydeposits films having a SWCNT degree of alignment of ±17° or better.This includes embodiments in which the SWCNTs have a degree of alignmentof ±16° or better, and further includes embodiments in which the SWCNTshave a degree of alignment off 15° or better.

The semiconductor-selective polymer that wraps the s-SWCNTs is presentby virtue of a highly selective pre-sorting of the s-SWCNT from astarting sample containing both s-SWCNTs and m-SWCNTs. Thesemiconductor-selective polymers selectively attach to (e.g., adsorb on)the surfaces of s-SWCNTs relative to the surfaces of m-SWCNTs. Thisallows for the separation of the selectively wrapped s-SWCNTs from them-SWCNTS using, for example, centrifugation and filtration. Bypre-sorting the SWCNTs to remove m-SWCNTs, films having very highs-SWCNT purity levels can be formed, where s-SWCNT purity level refersto the percentage of SWCNTs in the film that are s-SWCNTs. For example,some of the films formed by the dose-controlled, floating evaporativeself-assembly method have a s-SWCNT purity level of at least 99%. Thisincludes films having a s-SWCNT purity level of at least 99.5% andfurther includes films having a s-SWCNT purity level of at least 99.9%.

A number of semiconductor-selective polymers are known. Description ofsuch polymers can be found, for example, in Nish, A.; Hwang, J. Y.;Doig, J.; Nicholas, R. J. Highly selective dispersion of single-walledcarbon nanotubes using aromatic polymers. Nat. Nanotechnol. 2007, 2,640-6. The semiconductor-selective polymers are typically organicpolymers with a high degree of π-conjugation and include polyfluorenederivatives, such as poly(9,9-dialkyl-fluorene) derivatives, andpoly(phenyl vinylene) derivatives. While the semiconductor-selectivepolymers may be conductive or semiconductive polymers, they can also beelectrically insulating.

The organic solvent desirably has a relatively low boiling point at thefilm depositions temperature and pressure, typically ambient temperatureand pressure, such that it evaporates rapidly. In addition, it shouldhave the capacity to solubilize thesemiconductor-selective-polymer-wrapped s-SWCNTs. Examples of suitableorganic solvents include chloroform, dichloromethane,N,N-dimethylformamide, benzene, dichlorobenzene, toluene and xylene.

The dose of liquid solution that contains the organic solvent and thesolvated polymer-wrapped s-SWCNTs is a quantity of liquid, such as adroplet, having a volume much smaller than that of the aqueous medium towhich it is delivered. By using very small volume doses to deliver thes-SWCNTs to a substrate, the present methods are able to form highdensity films with very small quantities of SWCNTs and organic solvents,relative to other solution-based SWCNT deposition methods. By way ofillustration only, the doses used in the present methods may have avolume in the range from about 0.5 to about 50 μl. However, volumesoutside of this range can be used. The concentration of SWCNTs in eachdose can be adjusted to control the density of s-SWCNTs in a depositedstripe. If a plurality of stripes are deposited, the concentration ofs-SWCNTs in different doses can be the same or different. By way ofillustration only, the doses used in the present methods may have aSWCNT concentration in the range from about 1 to about 50 μg/ml.However, concentrations outside of this range can be used. The dosedispensation rate can be adjusted to control the periodicity of thestripes formed on a substrate. If a plurality of stripes are deposited,the dose dispensation rate can be kept constant throughout the method toprovide regularly spaced stripes on a substrate. Alternatively, the dosedispensation rate can be changed throughout the method to providestripes having different inter-stripe spacings.

The substrate onto which the films comprising alignedsemiconductor-selective-polymer-wrapped s-SWCNTs are deposited aresufficiently hydrophobic that the polymer-wrapped s-SWCNTs have a higheraffinity for the substrate than the aqueous medium. The hydrophobicsubstrates can be composed of a hydrophobic material or can comprise ahydrophobic surface coating over a support substrate. Hydrophobicpolymers are examples of materials that can be used as substratematerials, including coatings. If the films are to be used as a channelmaterial in an FET, the substrate can comprise a gate dielectricmaterial, such as SiO₂, coated with a hydrophobic coating.

FETs comprising the films of aligned s-SWCNTs as channel materialsgenerally comprise a source electrode and a drain electrode inelectrical contact with the channel material; a gate electrode separatedfrom the channel by a gate dielectric; and, optionally, an underlyingsupport substrate. Various materials can be used for the components ofthe FET. Example 2, below, illustrates an FET that includes a channelcomprising a film comprising aligned s-SWCNTs, a SiO₂ gate dielectric, adoped Si layer as a gate electrode and metal (Pd) films as source anddrain electrodes. However, other materials may be selected for each ofthese components. Channel materials comprising the highly aligneds-SWCNTs having high s-SWCNT purity levels and high SWCNT density areable to provide FETs characterized by both high on-conductance per width(G_(ON)/W (μS/μm)) and high on/off ratios. For example, some embodimentsof the FETs have an on-conductance per width of at least 5 μS μm⁻¹ andan on/off ratio per width of at least 1×10⁵. This includes FETs havingon-conductance per width greater than 7 μS μm⁻¹ and an on/off ratio perwidth of at least 1.5×10⁵ and further includes FETs havingon-conductance per width greater than 10 μS μm⁻¹ and an on/off ratio perwidth of at least 2×10⁵. These performance characteristics can beachieved with channel lengths in the range from, for example, about 400nm to about 9 μm, including channel lengths in the range from about 1 μmto about 4 μm. For example, as illustrated in Example 2, below, someembodiments of the FETs having a channel length of no greater than about400 nm are able to provide on-conductance per width greater than 60 μSμm⁻¹ and an on/off ratio per width of at least 2×10⁵, while someembodiments of the FETs having a channel length of at least about 9 μmare able to provide on-conductance per width greater than 7 μS μm⁻¹ andan on/off ratio per width of at least 2×10⁶.

EXAMPLES Example 1

In this example, arrays of parallel stripes comprising aligned s-SWCNTswith exceptional electronic-type purity levels (99.9% s-SWCNTs) weredeposited at high deposition velocities using the dose-controlled,floating evaporative self-assembly process with control over theplacement of the stripes and the quantity of s-SWCNTs.

By decoupling the s-SWCNT stripe formation from the evaporation of thebulk solution and by iteratively applying the s-SWCNTs in controlleddoses, the dose-controlled, floating evaporative self-assembly processformed stripes in which the s-SWCNTs were aligned within ±14°, werepacked at a density of −50 s-SWCNTs μm⁻¹, and constituted primarily awell-ordered monodispersed layer. FET devices incorporating the stripesshowed high performance with a mobility of 38 cm²v⁻¹s⁻¹ and on/off ratioof 2.2×10⁶ at a 9 μm channel length.

Results And Discussion

Two different types of s-SWCNT inks were examined. The first type of inkwas processed from arc-discharge SWCNT powders (Nano Lab, Inc.). In thiscase, the polyfluorene derivativepoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6′-{2,2′-bipyridine})](PFO-BPy), was employed as a semiconductor-selective polymer. PFO-BPyhas been shown to selectively wrap highly semiconducting SWCNT species.(See, Mistry, K. S.; Larsen, B. A.; Blackburn, J. L. High-YieldDispersions of Large-Diameter Semiconducting Single-Walled CarbonNanotubes with Tunable Narrow Chirality Distributions. ACS Nano 2013, 7,2231-2239.) The arc-discharge powder and PFO-BPy were dispersed intoluene by ultrasonication where the PFO-BPy wrapped s-SWCNTs weresolubilized, while leaving the remaining carbon residuals and m-SWCNTsin large bundles and aggregates, which were removed by centrifugation.Absorption spectra of the sorted and unsorted SWCNT solutions wereobtained for comparison. Metallic peaks present in the unsorted spectraaround 700 nm were absent after sorting with PFO-BPy. Following theinitial sorting process excess polymer chains were removed by repeateddispersion and centrifugation of the SWCNTs in tetrahydrofuran. Thesecond type of ink was processed from high pressure carbon monoxide(HiPco) produced powders (Nanointegris Inc.). In this case thepolyfluorene derivative poly[(9,9-di-n-octylfluorenyl-2,7-diyl)] (PFO)was used as a semiconductor-selective polymer. (See, Nish, A.; Hwang, J.Y.; Doig, J.; Nicholas, R. J. Highly selective dispersion ofsingle-walled carbon nanotubes using aromatic polymers. Nat.Nanotechnol. 2007, 2, 640-6.)

FIG. 1 is a schematic illustration of the method. A 2 μl dose of >99.9%purified arc discharge s-SWCNT ink in chloroform (concentration=10 μgml⁻¹) was dropped onto the water surface 0.5 cm away from the verticallyoriented substrate as shown in panel (i) of FIG. 1. The dose covered thewater surface by spreading at the air/water interface and reached thesubstrate quickly as a result of surface tension effects (panel (ii) inFIG. 1). Chloroform was chosen as a suitable solvent for this study asit spreads and evaporates rapidly across the water surface.

It should be noted that unlike previous studies on evaporativeself-assembly from aqueous solutions of SWCNTs where low pressure wasrequired to speed up the evaporation of water and hence the assemblyprocess, the use of high vapor pressure organic solvents in this exampleallows for much more rapid assembly under ambient conditions. Forexample, a deposition velocity of 5 mm min⁻¹ at ambient conditions isdemonstrated here. Reported deposition velocities using standardevaporative self-assembly from aqueous solution are much slower, only0.02 and 0.001 mm min⁻¹ at 70 and 760 Torr, respectively, using similarsubstrate dimensions. As the organic ink spreads, it comes in contactwith the partially submerged substrate. Subsequent rapid evaporation ofthe chloroform (FIG. 1, panel (ii)) results in the formation of a stripeof aligned s-SWCNTs on the vertically submerged substrate. As thesolvent level rapidly decreases during evaporation, the s-SWCNTs tend toorient perpendicular to the evaporation front, which is sterically amore favored position.

The results of these experiments showed the formation of continuousstripes of aligned s-SWCNTs (FIG. 1, panels (iii) and (iv)) anddemonstrated the ability to control three pivotal factors: (1) the widthof the stripes, (2) the density of SWCNTs within each stripe, and (3)the spacing between the stripes. Control over the width of the stripesby varying the substrate elevation velocity was demonstrated. For a doseconcentration of 10 μg ml⁻¹, at a high velocity of 9 mm min⁻¹ the SWCNTsbecame randomly disordered while at 1 mm min⁻¹ they began to aggregateinto large bundles or ropes. At an optimized velocity of 5 mm min⁻¹, thes-SWCNTs in the resulting stripes were well isolated from one-anotherand well aligned. FIG. 2 shows an optical micrograph of aligned s-SWCNTstripes with widths of 20 (±2.5) μm fabricated under these optimizedconditions. By optimizing the elevation velocity the width of thestripes could be dictated. Another crucial factor for scalableelectronics is controlling the stripe spacing. To demonstrate periodicstripe spacing, a constant substrate elevation velocity of 5 mm min⁻¹was set and one dose per 1.2 sec was applied to achieve a stripeperiodicity of 100 μm (FIG. 2). With this method it is possible tofabricate aligned SWCNT arrays with control over the stripe width,stripe periodicity, and SWCNT density, in a continuous manner, whichmakes this appealing for high throughput microelectronic applications.The substrates used here were treated with a Piranha solution of H₂O₂(33%)/concentrated H₂SO₄ (67%) followed by vapor deposition of ahexamethyldisilizane self-assembling monolayer to increase thehydrophobicity of the SiO₂ surface. Unlike previous studies onevaporative self-assembly from aqueous solutions of SWCNTs, which workedwell on hydrophilic substrates, this method using organic solutions gavethe best results on HMDS treated substrates. HMDS treated substrates maybe advantageous for TFT devices as they lead to lower charged impurityconcentrations.

In the higher resolution SEM and AFM images in FIGS. 3, 4(A) and 4(B),the degree of alignment of the arc-discharge s-SWCNTs was notably higherthan observed for Langmuir-Blodgett and spin casting methods. (See,LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim, J. M.; Bao, Z.Self-sorted, aligned nanotube networks for thin-film transistors.Science 2008, 321, 101-4 and Cao, Q.; Han, S. J.; Tulevski, G. S.; Zhu,Y.; Lu, D. D.; Haensch, W. Arrays of single-walled carbon nanotubes withfull surface coverage for high-performance electronics. Nat.Nanotechnol. 2013, 8, 180-6.) This is quantified in more detail viaRaman spectroscopy, below. SEM images were used to quantify a linearpacking density of 40-50 tubes μm⁻¹. The packing density achieved hereis in-between the relatively lower densities achieved from aqueousself-assembly (˜20 tubes μm⁻¹), and higher values achieved usingLangmuir-Blodgett and Langmuir-Schaefer methods (>100 tubes μm⁻¹). (See,Shastry, T. A.; Seo, J. W.; Lopez, J. J.; Arnold, H. N.; Kelter, J. Z.;Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Large-area,electronically monodisperse, aligned single-walled carbon nanotube thinfilms fabricated by evaporation-driven self-assembly. Small 2013, 9,45-51; Cao, Q.; Han, S. J.; Tulevski, G. S.; Zhu, Y.; Lu, D. D.;Haensch, W. Arrays of single-walled carbon nanotubes with full surfacecoverage for high-performance electronics. Nat. Nanotechnol. 2013, 8,180-6 and Li, X.; Zhang, L.; Wang, X.; Shimoyama, I.; Sun, X.; Seo, W.S.; Dai, H. Langmuir-Blodgett assembly of densely aligned single-walledcarbon nanotubes from bulk materials. J. Am. Chem. Soc. 2007, 129,4890-1.) In addition, the thickness of the stripes was <3 nm indicatingthat the s-SWCNTs were deposited as monolayers or bilayers of SWCNTsover the entire line section.

Polarized Raman spectroscopy was used to quantify s-SWCNT alignmentwithin each stripe. FIG. 5 shows a representative Raman spectrum fromthe arc-discharge s-SWCNTs. Present in the spectrum are the radialbreathing modes (RBM, near 160 cm⁻¹), D-band, G-band, and G′-band modes,and double resonance characteristics associated with the M-band (near1750 cm⁻¹). (See, Brar, V.; Samsonidze, G.; Dresselhaus, M.;Dresselhaus, G.; Saito, R.; Swan, A.; Ünlü, M.; Goldberg, B.; SouzaFilho, A.; Jorio, A. Second-order harmonic and combination modes ingraphite, single-wall carbon nanotube bundles, and isolated single-wallcarbon nanotubes. Phys. Rev. B 2002, 66, 155418.) The one-phonon siliconpeak at 519 cm⁻¹ and two phonon scattering peaks in the range of900-1100 cm⁻¹ were used for calibration and normalization. (See, Temple,P.; Hathaway, C. Multiphonon Raman Spectrum of Silicon. Phys. Rev. B1973, 7, 3685-3697.) The spectrum in the vicinity of the G-band from1500-1680 cm⁻¹ is plotted in FIG. 6 as a function of the angle, δ,between the polarization of the Raman excitation laser and the stripelong-axis. The intensity of the G-band versus δ is plotted in the inset.

The assumption was made that the orientation of the s-SWCNTs within thestripes is described by a Gaussian angular distribution,

$\begin{matrix}{{{f(\theta)} = {\exp\left( \frac{- \theta^{2}}{2\sigma^{2}} \right)}},} & (1)\end{matrix}$where, f is the probability of finding a s-SWCNT with its long-axismisaligned from the long-axis of the stripe by angle, θ, and σ is theangular width of the distribution. Based on this distribution and usingthe fact that the Raman G-band for a single s-SWCNT will follow a cos²dependence with the laser polarization, the G-band peak-to-valley ratiofor excitation polarized parallel to the stripe versus perpendicular toit, goes as,

$\begin{matrix}{r = {\frac{I_{G}\left( {\delta = {0{^\circ}}} \right)}{I_{G}\left( {\delta = {90{^\circ}}} \right)} = {\frac{\int_{{- \pi}/2}^{\pi/2}{{f(\theta)}{\cos(\theta)}^{2}\ {\partial\theta}}}{\int_{{- \pi}/2}^{\pi/2}{{f(\theta)}{\cos\left( {\theta - {90{^\circ}}} \right)}^{2}\ {\partial\theta}}}.}}} & (2)\end{matrix}$(See, Cao, Q.; Han, S. J.; Tulevski, G. S.; Zhu, Y.; Lu, D. D.; Haensch,W. Arrays of single-walled carbon nanotubes with full surface coveragefor high-performance electronics. Nat. Nanotechnol. 2013, 8, 180-6; Li,X.; Zhang, L.; Wang, X.; Shimoyama, I.; Sun, X.; Seo, W. S.; Dai, H.Langmuir-Blodgett assembly of densely aligned single-walled carbonnanotubes from bulk materials. J. Am. Chem. Soc. 2007, 129, 4890-1;Hwang, J.; Gommans, H.; Ugawa, A.; Tashiro, H.; Haggenmueller, R.;Winey, K. I.; Fischer, J. E.; Tanner, D.; Rinzler, A. Polarizedspectroscopy of aligned single-wall carbon nanotubes. DepartmentalPapers (MSE) 2000, 87 and Pint, C. L.; Xu, Y.-Q.; Moghazy, S.;Cherukuri, T.; Alvarez, N. T.; Haroz, E. H.; Mahzooni, S.; Doom, S. K.;Kono, J.; Pasquali, M. Dry contact transfer printing of aligned carbonnanotube patterns and characterization of their optical properties fordiameter distribution and alignment. ACS Nano 2010, 4, 1131-1145.)

The r for both types of s-SWCNTs aligned by this method was measured.For s-SWCNTs of diameter 0.8-1.1 nm produced by the HiPco method, r=15.8corresponding to σ=31°. For s-SWCNTs of diameter 1.3-1.7 nm produced bythe arc-discharge method, r=3.47 corresponding to σ=14.41°. The degreeof alignment of arc-discharge s-SWCNTs was significantly better thanHiPco s-SWCNTs likely because the arc-discharge s-SWCNTs were stiffer(due to their larger diameter). The average length of the arc-dischargeand HiPco s-SWCNTs were 464.6 and 449.1 nm, respectively, as determinedby AFM. The similarity in length suggests that the improved alignment ofthe arc-discharge SWCNTs may be solely a result of the structuralrigidity. Alignment imperfections existed in both HiPco- andarc-discharge s-SWCNT assemblies, which included voids, bending defects,and randomly oriented SWCNTs. However, defects due to bending andlooping of nanotubes were associated more with HiPco s-SWCNTs. Thedegree of alignment of the arc-discharge s-SWCNTs here is compared toother reported methods with comparable s-SWCNT densities in FIG. 8. Thedata in the figure are taken from Shastry 2013 (Shastry, T. A.; Seo, J.W.; Lopez, J. J.; Arnold, H. N.; Kelter, J. Z.; Sangwan, V. K.; Lauhon,L. J.; Marks, T. J.; Hersam, M. C. Large-area, electronicallymonodisperse, aligned single-walled carbon nanotube thin filmsfabricated by evaporation-driven self-assembly. Small 2013, 9, 45-51)Lemieux 2008 (LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim,J. M.; Bao, Z. Self-sorted, aligned nanotube networks for thin-filmtransistors. Science 2008, 321, 101-4), Cao 2013 (Cao, Q.; Han, S. J.;Tulevski, G. S.; Zhu, Y.; Lu, D. D.; Haensch, W. Arrays of single-walledcarbon nanotubes with full surface coverage for high-performanceelectronics. Nat. Nanotechnol. 2013, 8, 180-6), Shekhar 2011 (Shekhar,S.; Stokes, P.; Khondaker, S. I. Ultrahigh density alignment of carbonnanotube arrays by dielectrophoresis. ACS Nano 2011, 5, 1739-46), Engel2008 (Engel, M.; Small, J. P.; Steiner, M.; Freitag, M.; Green, A. A.;Hersam, M. C.; Avouris, P. Thin Film Nanotube Transistors Based onSelf-Assembled, Aligned, Semiconducting Carbon Nanotube Arrays. ACS Nano2008, 2, 2445-2452) and Hong 2010 (Hong, S. W.; Banks, T.; Rogers, J. A.Improved Density in Aligned Arrays of Single-Walled Carbon Nanotubes bySequential Chemical Vapor Deposition on Quartz. Adv. Mater. 2010, 22,1826-1830).

The high electronic-grade purity and the high degree of alignment areboth attractive for electronic devices based on s-SWCNTs. As aproof-of-principal, s-SWCNT FETs were fabricated and their chargetransport mobility and conductance modulation were evaluated. FIG. 9shows the output characteristics of a typical 9 μm channel length device(FIG. 11) that has p-type behavior, which is expected for CNT FETsmeasured in atmosphere. The transfer characteristics shown in FIG. 10,demonstrate the excellent semiconducting nature of the aligned SWCNTs.At a source-drain bias, K_(DS), =−1 V, the conductance per width andon/off conductance modulation are 4.0 μS μm⁻¹ and 2.2×10⁶, respectively(FIG. 10). The field-effect mobility was extracted using a parallelplate capacitance model, according to μ=gL/V_(DS)C_(ox)W, where g is thetransconductance and L and W are the channel length and width,respectively. The mobility achieved here was 38 cm²V⁻¹s⁻¹. Theperformance of the FETs were compared to those of Sangwan et al., whoachieved simultaneous high on/off ratio and conductance modulation.(See, Sangwan, V. K.; Ortiz, R. P.; Alaboson, J. M. P.; Emery, J. D.;Bedzyk, M. J.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. FundamentalPerformance Limits of Carbon Nanotube Thin-Film Transistors AchievedUsing Hybrid Molecular Dielectrics. ACS Nano 2012, 6, 7480-7488.)Compared to the 10 μm channel length devices of Sangwan et al., asimilar on-conductance per width was achieved but over 10× higher on/offconductance modulation. Sangwan et al. achieved on/off conductancemodulation similar to that reported here at a channel length of ˜50 μm;however, the on-conductance per width dropped to 10× smaller than thatfor the present FET.

In conclusion, well-aligned arrays of highly electronic-type sortedsemiconducting single-walled carbon nanotubes (s-SWCNTs) were fabricatedusing dose-controlled, floating evaporative self-assembly, by exploitingthe spreading of chloroform at the air/water interface of a trough. Therapidly evaporating chloroform front aided in the alignment of the tubesunder ambient conditions, on the partially submerged substrate at theair/water interface. The use of timed doses of predetermined aliquots ofsolution to control the position and/or periodicity of the stripes makesthis an attractive cost-effective large area fabrication process forcreating functional s-SWCNT architectures.

Experimental Section Preparation of Semiconducting SWCNTs

Arc-Discharge:

Mixtures of arc-discharge SWCNT powders (2 mg ml¹) and PFO-BPy (AmericanDye Source, 2 mg ml¹) were sonicated for 30 min in toluene (30 ml). Thesolution was centrifuged in a swing bucket rotor at 50,000 g for 5 min,and again at 50,000 g for 1 hr. The supernatant was collected andfiltered through a syringe filter. A distillation removed toluene over a30 min duration. The residue of PFO-BPy and s-SWCNTs were redispersed intetrahydrofuran (THF). The s-SWCNT solution in THF was centrifuged at atemperature of 15° C. for 12 hours. The supernatant (excess PFO-BPy) wasdiscarded and the pellet was redispersed into THF. After removing theTHF, the residue was dispersed in chloroform to a concentration of 10 μgml¹.

HiPco:

The initial dispersion of HiPco (Nanointegris Inc.) SWCNTs were preparedusing 2 mg ml¹ of HiPco powder and 2 mg ml¹ of PFO (American Dye Source)in toluene. The same sonication, centrifugation, and distillationprocedures as the arc-discharge SWCNTs were used for the dispersion ofs-SWCNTs, separation of unwanted material, and removal of excesspolymer.

Raman Spectroscopy Characterization:

Raman characterization was measured in a confocal Raman microscope withlaser excitation wavelength of 532 nm (Aramis Horiba Jobin Yvon ConfocalRaman Microscope.). The device was equipped with a linear polarizingfilter between the sample and the incident beam laser to allowpolarization-dependent measurements.

Imaging:

SEM images were collected with LEO-1530 field-emission scanning electronmicroscope (FE-SEM). The surface morphology of the s-SWCNTs was imagedusing a Nanoscope III Multimode atomic force microscope (DigitalInstruments). Tapping mode was utilized for the AFM measurement. Atriangular cantilever with an integral pyramidal Si₃N₄ tip was used. Thetypical imaging force was of the order of 10⁻⁹N.

Langmuir-Blodgett trough and substrate: The LB trough (KSV NIMA Mediumsize KN 2002) was primarily used as a trough to spread s-SWCNTs at 23°C. with Wilhelmy balance (Platinum plate). Milli Q water (resistivelyca. 18.2 MΩ cm) was used as the water sub-phase. The Si/SiO₂ substrateswere cleaned by a Piranha solution of H₂O₂ (33%)/concentrated H₂SO₄(67%) for 20 min and rinsed with deionized (DI) water. After Piranhatreatment, the substrates were covered by a hexamethyldisilizaneself-assembling monolayer (vapor deposition).

FET Fabrication:

First, stripes of arc-discharge s-SWCNTs were deposited on a highlydoped Si substrate with a 90 nm thermally grown SiO₂, which served asthe backgate electrode and dielectric, respectively. Electron beamlithography was then used to pattern the stripes so that they hadwell-defined widths of 4 μm. Samples were then annealed in a mixture of≧99.999% Ar (95%): H₂ (5%) in order to partially degrade the PFO-BPypolymer, followed by annealing in vacuum at 1×10⁻⁷ Torr and 400° C. for20 min. A second electron beam lithography step was used to define thetop-contacted electrodes. Thermal deposition of Pd (40 nm) was used tocreate source and drain contacts to the s-SWCNT stripe. Finally, thedevices were annealed in argon atmosphere at 225° C.

Example 2

This example illustrates the performance of exceptionallyelectronic-type sorted, aligned s-SWCNTs in field effect transistors.High on-conductance and high on/off conductance modulation aresimultaneously achieved at channel lengths that are both shorter andlonger than individual s-SWCNTs. The s-SWCNTs were isolated fromheterogeneous mixtures of s-SWCNTs and m-SWCNTs using apolyfluorene-derivative as a semiconductor selective agent and alignedon substrates via dose-controlled, floating evaporative self-assembly.

Example 1 illustrates that s-SWCNTs isolated from polydisperse mixturesof SWCNTs using the polyfluorene-derivativepoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6′-{2,2′-bipyridine})](PFO-BPy) as a sorting agent can be aligned on substrates viadose-controlled, floating evaporative self-assembly. This exampleevaluates the performance of these aligned s-SWCNTs as channel materialsin FETs and reports exceptionally high on/off conductance modulation andon-conductance, over a range of channel lengths, compared to previousreported studies.

High purity s-SWCNTs were extracted from an as-produced arc dischargesynthesized SWCNT powder purchased from NanoLab Inc. s-SWCNTs wereisolated by dispersing the powder in a solution of PFO-BPy in tolueneadapting procedures reported by Mistry et al. During the initialdispersion process the polymer selectively wraps and solubilizespredominantly semiconducting species. Excess polymer was removed bydispersing the s-SWCNTs in tetrahydrofuran followed by repeatedsedimentation and dispersion cycles, which effectively removed polymerchains that were not tightly bound to the s-SWCNT surface. Removingexcess polymer improved the self-assembly of aligned s-SWCNT arrays andimproved the contact of s-SWCNT to metal electrodes in FETs for higherperformance.

Optical absorption spectra of sorted and unsorted PFO-BPy SWCNTsolutions were obtained for comparison. In both the sorted and unsortedspectra the S₂₂ optical transitions were broadened around a wavelengthof 1050 nm due to the overlap of peaks from a chiral distribution ofs-SWCNTs with diameters 1.3-1.7 nm. Previous investigations have usedphotoluminescence and Raman spectroscopy to confirm that PFO-BPydifferentiates by electronic type, but not strongly by diameter, thusresulting in diameter distributions that match the s-SWCNT startingmaterial. The broad M₁₁ peak, which is visible in the unsorted spectrais immeasurable after sorting suggesting a purity of >99%. The featuresfrom 400-600 nm are combinations of the S₃₃ s-SWCNT transitions coupledwith absorption from the PFO-BPy, which is centered at 360 nm.

For the FET devices, highly doped Si substrates with a 90 nm thick SiO₂dielectric layer were used as the gate electrode and dielectric,respectively. Prior to s-SWCNT deposition, the substrates were treatedwith a solution of 20 ml H₂SO₄:10 ml H₂O₂ followed by vapor depositionof a hexamethyldisilizane self-assembling monolayer to increase thehydrophobicity of the SiO₂ surface. The dose-controlled, floatingevaporative self-assembly procedure is described briefly below and indetail in Example 1, above. Droplets (“doses”) of a solution of 10 μgml⁻¹ s-SWCNTs in chloroform were cast on a water trough. The s-SWCNTsspread across the surface of the water and deposited on the substratewhich was slowly extracted from the trough, normal to the air-waterinterface. Each droplet created a well-aligned stripe of s-SWCNTs acrossthe entire width of the substrate. Here, periodic arrays of stripes wereachieved by successively adding droplets to the through surface at 12second intervals as the substrate was elevated at a constant rate of 5mm min⁻¹.

The uniformity, density, and thickness of s-SWCNTs in a single stripewere characterized using SEM, AFM and Raman spectroscopy (FIGS. 12, 13and 14, respectively). The AFM thickness profile shown in FIG. 13 variesbetween 2-4 nm, indicating for the most part that each stripe iscomposed of single layers of s-SWCNTs with localized regions ofinter-nanotube overlap. The G-band Raman intensity (3 mW, 532 nm) of thes-SWCNTs is spatially mapped for one stripe in FIG. 14, normalized tothe substrate Si peak intensity. The G-band intensity varies only by±12.5% indicating that the density of the s-SWCNTs within each stripewas fairly uniform. SEM images (example presented in FIG. 12) were usedto quantify a s-SWCNT linear packing density of 40-50 SWCNTs μm⁻¹. Thesemeasurements indicated that the stripes were monolayers of well-isolateds-SWCNTs with occasional inter-nanotube overlap from randomly orienteds-SWCNTs that interweave into the well-ordered s-SWCNTs at a linearoccurrence of one randomly oriented s-SWCNT per two micrometers. Therandomly overlapping nanotubes may be beneficial in establishinginter-SWCNT connectivity in the FET percolation regime where channellength (L_(C)) is much greater than the s-SWCNT length (L_(C)>>L_(N)).

FETs were fabricated from the stripes using electron beam (e-beam)lithography. The stripes varied in width from 10-20 μm. Therefore, theywere lithographically patterned to ensure a consistent FET channel widthof 4 μm. First, e-beam patterning was used to expose regions around thes-SWCNT stripes where unwanted s-SWCNTs were to be removed and etchingvia a 20 s exposure to an oxygen plasma (50 W, 10 mTorr, and 10 sccm O₂flow rate) was used. To remove PMMA resist, films were developed inacetone and toluene each for 30 minutes at 60° C. and rinsed inisopropyl alcohol. Samples were then annealed in ≧99.999% Ar (95%):H₂(5%) at 500° C. to partially remove and decompose the PFO-BPy. Anadditional annealing step was conducted in high vacuum at 1×10⁻⁷ Torr at400° C. for 20 minutes to further degrade and partially remove polymerresidue. A second e-beam step defined the source-drain electrodes andcontact pads. The pattern was developed to remove resist and theunderlying electrode pattern was exposed to ultraviolet light in air ata power of 0.1 W cm⁻² for 90 s (SCD88-9102-02 BHK Inc.) to improveadhesion of Pd to the s-SWCNT surface. Thermal evaporation of a 40 nmthick layer of Pd defined the source-drain electrodes, followed by liftoff, which was achieved by soaking samples in acetone at 120° C. for 5minutes and bath sonication in acetone for 30 seconds Immediately beforemeasurement the devices were annealed at 225° C. in Ar to improvecontact resistance. The resulting device architecture of a 400 nm deviceis shown in FIG. 12.

The electronic characteristics of s-SWCNT channel FETs were measuredusing a Keithley source meter instrument (Model 2636A). Measurementswere made on devices of varying channel length in order to quantifytransport properties in both the direct and percolative regimes and toassess the electronic-type purity of the s-SWCNTs. The characteristicsof a typical 9 μm channel length device (in the percolating regime) areshown in FIG. 15, demonstrating the p-type behavior of the aligneds-SWCNTs. At low fields the current output followed linear behaviorindicating ohmic contact at the Pd-SWCNT interface. The transfercharacteristics are shown in the insert. Hysteresis typical ofSWCNT-based devices that are not treated to remove the surfaceadsorbates was observed. The on/off ratio of the device was 5×10⁵. Theon-conductance was 7.3 μS μm⁻¹, which corresponds to a field-effectmobility of 46 cm²V⁻¹s⁻¹, applying a standard parallel plate capacitancemodel:

$\begin{matrix}{\mu = {\left( \frac{L_{C}t_{ox}}{\varepsilon_{{SiO}_{2}V_{SD}W}} \right){\left( \frac{\mathbb{d}I}{\mathbb{d}V_{g}} \right).}}} & (3)\end{matrix}$

The on- and off-conductance normalized to width for each device andvarying channel lengths of 0.4, 1, 2, 3, 4 and 9 μm are plotted in FIG.17. The champion devices in order of increasing channel length haveon-conductance per width of 61, 31, 24, 18, 16, and 7.5 μS μm⁻¹. Thehighest on/off ratios achieved in order of increasing channel length are4.1×10⁵, 4.1×10⁵, 4.4×10⁵, 5.6×10⁵, 3.4×10⁵, and 2.2×10⁷. Thedistribution of on-conductance and on/off ratio of all of the devicestested are compared in FIG. 18 with the performance state-of-the-arts-SWCNT FETs reported in the literature. The prior studies are Sangwan(V. K. Sangwan, R. P. Ortiz, J. M. P. Alaboson, J. D. Emery, M. J.Bedzyk, L. J. Lauhon, T. J. Marks, and M. C. Hersam, ACS Nano 6 (8),7480 (2012)), Engel (M. Engel, J. P. Small, M. Steiner, M. Freitag, A.A. Green, M. S. Hersam, and P. Avouris, ACS Nano. 2 (12), 2445 (2008)),Miyata (Y. Miyata, K. Shiozawa, Y. Asada, Y. Ohno, R. Kitaura, T.Mizutani, and H. Shinohara, Nano. Research 4 (10), 963 (2011)), Kang (S.J. Kang, C. Kocabas, T. Ozel, M. Shim, N. Pimparkar, M. A. Alam, S. V.Rotkin, and J. A. Rogers, Nat. Nanotechnol. 2 (4), 230 (2007)), Jin (S.H. Jin, S. N. Dunham, J. Song, X. Xie, J. H. Kim, C. Lu, A. Islam, F.Du, J. Kim, J. Felts, Y. Li, F. Xiong, M. A. Wahab, M. Menon, E. Cho, K.L. Grosse, D. J. Lee, H. U. Chung, E. Pop, M. A. Alam, W. P. King, Y.Huang, and J. A. Rogers, Nat. Nanotechnol. 8 (5), 347 (2013)), Cao (Q.Cao, S. J. Han, G. S. Tulevski, Y. Zhu, D. D. Lu, and W. Haensch, Nat.Nanotechnol. 8 (3),180 (2013)), Sun (D. M. Sun, M. Y Timmermans, Y.Tian, A. G. Nasibulin, E. I. Kauppinen, S. Kishimoto, T. Mizutani, andY. Ohno, Nat. Nanotechnol. 6 (3), 156 (2011)), Wu (J. Wu, L. Jiao, A.Antaris, C. L. Choi, L. Xie, Y. Wu, S. Diao, C. Chen, Y. Chen, and H.Dai, Small, n/a (2013)), and Kim (B. Kim, S. Jang, P. L. Prabhumirashi,M. L. Geier, M. C. Hersam, and A. Dodabalapur, App. Phys. Lett. 103 (8),082119 (2013)).

On-conductance as high as 240 μS μm⁻¹ has been reported for SWCNT FETsin the direct transport regime, however, the on/off ratio in suchdevices was limited to ≦10³, presumably by the presence of metallicnanotubes. Similarly, in the percolative regime, high on/off ratios onthe order of 10⁷ have been achieved, but devices were limited by anon-conductance of ≦4 μS μm⁻¹ for channel length of >5 μm. Due to thenumber of percolation pathways in network s-SWCNT FETs or the lowdensity of s-SWCNTs in aligned CVD films. For example, it has beenchallenging to achieve high on-conductance and on/off ratiosimultaneously in the percolative regime. (See, D. M. Sun, M. Y.Timmermans, Y. Tian, A. G. Nasibulin, E. I. Kauppinen, S. Kishimoto, T.Mizutani, and Y. Ohno, Nat. Nanotechnol. 6 (3), 156 (2011) and S. H.Jin, S. N. Dunham, J. Song, X. Xie, J. H. Kim, C. Lu, A. Islam, F. Du,J. Kim, J. Felts, Y. Li, F. Xiong, M. A. Wahab, M. Menon, E. Cho, K. L.Grosse, D. J. Lee, H. U. Chung, E. Pop, M. A. Alam, W. P. King, Y.Huang, and J. A. Rogers, Nat. Nanotechnol. 8 (5), 347 (2013).)

Here, high on-conductance and high on/off ratio were achievedsimultaneously. At a channel length of 400 nm in the direct regime, anon-conductance per width as high as 61 μS μm⁻¹ was achieved, whilemaintaining a median on/off ratio of 2×10⁵. At a channel length of 9 μmin the percolative regime, a median on/off ratio of 2×10⁶ was achieved,while reaching conductance values as high as 7.5 μS μm⁻¹. Atintermediate channel lengths ranging from 1-4 μm the achievedon-conductance and on/off ratio fell in between these two cases along aninversely sloping line in FIG. 18. In general, the performance of thepresent devices pushes out further to the upper-right corner of thegraph in FIG. 18 than any of the other families of devices, extendingfurther in on-conductance and on/off ratio than these previous studies.

The simultaneously high on-conductances and on/off ratios likelyoriginate from a combination of factors including (a) the highsemiconducting purity of the s-SWCNTs and (b) their high degree ofalignment. An additional factor may be attributed to (c) a reduction ininter-nanotube charge screening due to the presence of the polymerwrapper during s-SWCNT deposition which reduces s-SWCNT bundling andinteractions. To test factor (a), 22 different FET devices with channellengths of 400 nm were measured. The analysis of the individual s-SWCNTlength distribution presented in Example 1 indicated that roughly halfof the individual s-SWCNTs were longer than 400 nm. Thus, these 400 nmFETs provided a sensitive measure of the presence of metallic SWCNTs.These FETs were composed of 4,071 individual s-SWCNTs or small pairs orbundles of s-SWCNTs. The conductance modulation of each FET (FIG. 16)was used to estimate the electronic-type purity of the SWCNTs within theFET, where the presence of even a single metallic nanotube directlyspanning the channel will cause a substantial increase in theoff-conductance on the order of 1-10 μS. The median on-conductance was130 μS, therefore an on/off ratio for a FET containing a single metallicnanotube that spans the channel was expected to be reduced to the orderof 10¹-10². Of the devices measured, the lowest on/off ratio was6.8×10³, nearly two orders of magnitude larger than the highest on/offratio expected for a device containing a single metallic nanotube. Themedian on/off ratio was 2×10⁵ with only three devices having on/offratios lower than 10⁴. If half of the 4,071 species were individualSWCNTs that fully span the channel, which is a reasonable estimate basedon the measurements of length distributions in Example 1, thesemiconducting purity of the sorted SWCNTs here was ≧99.95%.

While factor (b) was confirmed by the SEM images, factor (c) wasassessed as follows. The average inter-SWCNT spacing was ˜20 nm, and thepresence of bundles in solution was minimal due to the high solubilityof PFO-BPy s-SWCNTs in chloroform. The stability of the PFO-BPy wrapperaround s-SWCNTs may limit inter-SWCNT interactions during drying therebycreating aligned films of s-SWCNTs with less charge screeninginteractions.

Under the assumption that the individual s-SWCNTs or small pairs orbundles of s-SWCNTs were all individual s-SWCNTs, a conductance as highas 1.2 μS per s-SWCNT was achieved in the devices of channel length 400nm. There are several factors that can be tailored to further improvethe conductance per tube such as (i) narrowing the diameter distributionand shifting towards larger diameter s-SWCNTs, (ii) sorting of thes-SWCNTs by length to ensure that they all actually do individually spanthe channel or alternatively using shorter channel lengths, (iii)implementing a local top gate structure for improved conductancemodulation, and (iv) better removal of the PFO-BPy polymer residualswhich may increase contact resistance at the SWCNT-Pd interface.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended here and their equivalents.

What is claimed is:
 1. A method of forming a film of aligned s-SWCNTs ona substrate, the method comprising: (a) partially submerging ahydrophobic substrate in an aqueous medium; (b) applying a dose of aliquid solution to the aqueous medium, the liquid solution comprisingsemiconductor-selective-polymer-wrapped s-SWCNTs dispersed in an organicsolvent, whereby the liquid solution spreads into a layer on the aqueousmedium at an air-liquid interface andsemiconductor-selective-polymer-wrapped s-SWCNTs from the layer aredeposited as a stripe of aligned semiconductor-selective-polymer-wrappeds-SWCNTs on the hydrophobic substrate; and (c) at least partiallywithdrawing the hydrophobic substrate, upon which the stripe of alignedsemiconductor-selective-polymer-wrapped s-SWCNTs has been deposited,from the aqueous medium.
 2. The method of claim 1, further comprisingrepeating steps (b) and (c) in sequence one or more times to deposit oneor more additional stripes of alignedsemiconductor-selective-polymer-wrapped s-SWCNTs on the hydrophobicsubstrate.
 3. The method of claim 1, further comprising removing thesemiconductor-selective polymer from the alignedsemiconductor-selective-polymer-wrapped s-SWCNTs.
 4. The method of claim1, wherein the semiconductor-selective-polymer-wrapped s-SWCNTs in thestripe have a degree of alignment of about ±15° or better.
 5. The methodof claim 1, wherein the single-walled carbon nanotube linear packingdensity in the stripe is at least 40 single-walled carbon nanotubes/μm.6. The method of claim 2, wherein the substrate is withdrawn at a rateof at least one mm/min.
 7. The method of claim 6, wherein stripes aredeposited on the substrate with a periodicity of 200μm or smaller. 8.The method of claim 1, wherein the semiconductor-selective polymer is apolyfluorene derivative.
 9. The method of claim 1, wherein the film hasa s-SWCNT purity level of at least 99%.
 10. The method of claim 1,wherein the semiconductor-selective -polymer-wrapped s-SWCNTs in thestripe have a degree of alignment of about ±17° or better.
 11. Themethod of claim 1, wherein the single-walled carbon nanotube linearpacking density in the stripe is at least 30 single-walled carbonnanotubes/μm.
 12. The method of claim 2, wherein the substrate iswithdrawn at a rate of at least five mm/min.
 13. The method of claim 1,wherein the organic solvent comprises chloroform, dichloromethane,N,N-dimethylformamide, benzene, dichlorobenzene, toluene or xylene. 14.The method of claim 1, wherein the hydrophobic substrate comprises SiO₂coated with a hydrophobic polymer.
 15. The method of claim 1, whereinthe organic solvent is chloroform.